The present invention relates to composite metal structure sin which a metal cladding having a lower density and a lower tensile strength at high temperature is reinforced by a core of a metal present in higher volume fraction and having both higher tensile strength and higher density than that of the cladding. The invention further relates to the reinforcement of lower density metal clad structures having a niobium titanium base cladding and a higher oxidation resistance, with metal core elements having a lower oxidation resistance as well as higher density and higher strength.
The invention additionally relates to body centered cubic metal structures in which a metal cladding having a lower density and a lower tensile strength at high temperature is reinforced by core elements of a metal present in higher volume fraction and having both higher tensile strength and higher density than that of the cladding. Lastly, the invention relates to metal-metal composite structures in which a lower density metal clad having a niobium titanium base and a higher oxidation resistance is reinforced with denser, but stronger, niobium base metal reinforcing elements having a lower oxidation resistance.
It is known that niobium base alloys have useful strength in temperature ranges at which nickel and cobalt base superalloys begin to show incipient melting. This incipient melting temperature is in the approximately 2300.degree. to 2400.degree. F. range. The use of the higher melting niobium base metals in advanced jet engine turbine hot sections would allow higher metal temperatures than are currently allowed. Such use of the niobium base alloy materials could permit higher flame temperatures and would also permit production of greater power at greater efficiency. Such greater power production at greater efficiency would be at least in part due to a reduction in cooling air requirements.
The commercially available niobium base alloys have high strength and high density but have very limited oxidation resistance in the range of 1600.degree. F. to 2400.degree. F. Silicide coatings exist which might offer some protection of such alloys at temperatures up to 2400.degree. F., but such silicide coatings are brittle enough that premature failure of the coating could be encountered where the coated part is highly stressed. The commercially available niobium base alloys also have high densities ranging from a low value of 8.6 grams per cubic centimeter for relatively pure niobium to values of about 10 grams per cubic centimeter for the strongest alloys.
Certain alloys having a niobium-titanium base have much lower densities of the range of 6-7 grams per cubic centimeter. A group of such alloys are the subject matter of commonly owned U.S. Pat. Nos. 4,956,144; 4,990,308; 5,006,307; 5,019,334; and 5,026,522. Such alloys can be formed into parts which have significantly lower weight than the weight of the presently employed nickel and cobalt superalloys as these superalloys have densities ranging from about 8 to about 9.3 grams per cubic centimeter. One of these patents, U.S. Pat. No. 5,019,334, concerns an alloy having the following composition in atom percent:
______________________________________ Concentration Ingredient Range ______________________________________ niobium balance titanium 31-48% aluminum 8-21% ______________________________________
A number of additional niobium based alloys are also the subject of commonly owned U.S. Patents. These are U.S. Pat. Nos. 4,890,244; 4,931,254; has uniquely valuable sets of properties but has densities which are higher than those of the other alloys. Commonly owned U.S. Pat. No. 4,904,546 concerns an alloy system in which a niobium base alloy is protected from environmental attack by a surface coating of an alloy highly resistant to oxidation and other atmospheric attack. Commonly owned copending applications, listed in the cross-reference section above pertain principally to matrix type composites in which reinforcement and matrix alloys are intimately intermixed and the relevant reinforcement ratios, as defined below, is above 50 and preferably above 100. By contrast the composites of the invention have reinforcement ratios below 50.
In devising alloy systems for use in aircraft engines the density of the alloys is, of course, a significant factor which often determines whether the alloy is the best available for use in the engine application. The nickel and cobalt based superalloys also have much greater tolerance to oxygen exposure than the commercially available niobium based alloys. The failure of a protective coating on a nickel or cobalt superalloy is a much less catastrophic event than the failure of a protective coating on many of the niobium based alloys and particularly the commercially available niobium based alloys. The oxidation resistance of the niobium based alloys of the above commonly owned patents is intermediate between the resistance of commercial Nb base alloys and that of the Ni- or Co-based superalloys.
While the niobium based alloys of the above commonly owned patents are stronger than wrought nickel or cobalt based superalloys at high temperatures, they are much weaker than cast or directionally solidified nickel or cobalt based superalloys at these higher temperatures. However, for many engine applications, structures formed by wrought sheet fabrication are used, since castings of sheet structures cannot be produced economically in sound form for these applications.
The advantage of use of niobium based structures is evidenced by the fact that the niobium based alloys can withstand 3 ksi for 1000 hours at temperatures of 2100.degree. F. The nickel and cobalt based wrought superalloys, by contrast, can withstand 3 ksi of stress for 1000 hours at only 1700.degree. to 1850.degree. F.
What is highly desirable in general for aircraft engine use is a structure which has a combination of lower density, higher strength at higher temperatures, good ductility at room temperature, and higher oxidation resistance. We have devised metal-metal clad composite structures which have such a combination of properties.
A number of articles have been written about use of refractory metals in high temperature applications. These articles include the following:
(1) Studies of composite structures of tungsten in niobium were performed at Lewis Research Center by D. W. Petrasek and R. H. Titran and are reported in a report entitled "Creep Behavior of Tungsten/Niobium and Tungsten/Niobium-1 Percent Zirconium Composites" and identified as Report No. DOE/NASA/16310-5 NASA TM-100804, prepared for Fifth Symposium on Space Nuclear Power Systems, University of New Mexico, Albuquerque, N.M. (Jan. 11-14 1988). No studies of reinforcing niobium base matrices with niobium base structures, nor the unique benefits of such reinforcing, is taught in this report.
(2) S. T. Wlodek, "The Properties of Cb-Ti-W Alloys, Part I", Oxidation, Columbium Metallurgy, D. Douglass and F. W. Kunz, eds., AIME Metallurgical Society Conferences, vol. 10, Interscience Publishers, New York (1961) pp. 175-204.
(3) S. T. Wlodek, "The Properties of Cb-AI-V Alloys, Part I", Oxidation, ibid., pp. 553-584.
(4) S. Priceman and L. Sama, "Fused Slurry Silicide Coatings for the Elevated Temperature Oxidation of CoIumbium Alloys", Refractory Metals and Alloys IV - TMS Conference proceedings, French Lick, IN, Oct. 3-5, 1965, vol. II, R. I. Jaffee, G. M. Ault, J. Maltz, and M. Semchyshen, eds., Gordon and Breach Science Publisher, New York (1966) pp. 959-982.
(5) M. R. Jackson and K. D. Jones, "Mechanical Behavior of Nb-Ti Base Alloys", Refractory Metals: Extraction, Processing and Applications, K. C. Liddell, D. R. Sadoway, and R. G. Bautista, eds., TMS, Warrendale, Pa. (1990) pp. 311-320.
(6) M. R. Jackson, K. D. Jones, S. C. Huang, and L. A. Peluso, "Response of Nb-Ti Alloys to High Temperature Air Exposure", ibid., pp 335-346.
(7) M. G. Hebsur and R. H. Titran, "Tensile and Creep Rupture Behavior of P/M Processed Nb-Base Alloy, WC-3009", Refractory Metals: State-of-the-Art 1988, P. Kumar and R. L. Ammon, eds., TMS, Warrendale, Pa. (1989) pp. 39-48.
(8) M. R. Jackson, P. A. Siemers, S. F. Rutkowski, and G. Frind, "Refractory Metal Structures Produced by Low Pressure Plasma Deposition", ibid., pp. 107-118.